I dont think so, I got some from work today 1tsp to every 50 gallons. super concentrated melafix but same thing as bettafix. I think with a major OD it can hurt the bacteria in the tank which might result in more injury/stress to fish.

I don't think it will be an issue. You probably know this, but I would suggest cleaning your tank out completely when you are done treating him. My Betta hated it, and was much happier after a good cleaning of the tank!

I don't bother with Bettafix. In my experiences it doesn't do a bit of good. Clean water and awuarium salt work far better.

Depends on how much of his fins he lost. Once the finrot is stopped and the betta is back to clean water the regrowth happens fairly quickly most of the time. A small 'edging' of finrot often only takes a few days to week before you'll see the clear regrowth putting the fin back where it was.
In more severe cases it can take several weeks... even a month or two. The two males I'm currently treating who came with some extreme finrot are going to be examples of the 'taking longer'. One of them has lost both his ventral fins. When the whole fin comes off like that, it generally takes at least a month to get it back. He's already has started to regrow them and you can see the clear fins starting to come back it.

I've read a lot about Bettafix. Some people claim that the active ingredient in it can harm Bettas, but the thing about it is that it is SOOO diluted in Bettafix that you really don't have to worry about it.

Unless you dumped the whole bottle in like a two gallon tank or something, I wouldn't concern myself with it at all.

If you DID do something like that, just change about half the water and replace it with non-bettafixed water.

Good luck!

I'm starting to think that mine had finrot when I got him and I didn't even know because I had never had Bettas before. I've had him for about 2 weeks - I didn't really 'treat' the finrot at all and it appears he has already grown a few milimeters of fin back. It's hard to tell, though. I am just guessing that from comparing him to pictures of him when I first got him.